In the processing of semiconductor wafers, such as silicon wafers, many techniques are known for modifying the surface of the wafer. These surface modifications may involve, for example, adding material, as in the case of a metal deposited on the wafer, or removing material, as is done during etching.
A particularly popular set of techniques for adding or removing material from the surface of a silicon wafer involves the use of plasma. A plasma is a gas (or gas mixture) which is energized so that it partially decomposes and forms a mixture of charged and uncharged particles. Plasmas may be generated by the application of an alternating current electrical signal (AC current) to the gas mixture. This generating current typically has a high frequency, usually in the radio frequency (RF) range, and is applied to the gas mixture by an electrode placed in the gas-containing vessel. The frequency of the electrical signal applied to the gas is called the RF drive frequency of the plasma processing system.
It has long been recognized in the art that the electrical response of plasmas generated by the application of AC power is nonlinear, that is, at typical applied power levels, the impedance (resistance to electrical current flow) of the plasma is not directly proportional to the applied voltage. The nonlinear response of the plasma to the applied power results in the generation of harmonic power frequencies in the plasma. Harmonic power frequencies are electrical signals with frequencies that are some integral multiple of the RF drive frequency of the plasma processing system. In a typical plasma processing system, hundreds of watts of power may be associated with harmonic power frequencies related to the RF drive frequency. These harmonics lead to a plasma that is not uniform in its characteristics.
When a non-uniform plasma is applied to a silicon wafer, for example, to etch the wafer, the non-uniform plasma results in a non-uniform wafer etch whose characteristics vary with the distance from the plasma electrode. FIG. 1 is an illustration depicting the non-uniformity of a plasma-etched wafer as a function of the distance from the center 16 of the wafer. In FIG. 1, the surface profile 12 of the wafer 10 is non-uniform and roughly sinusoidal. This non-uniform wafer processing may be undesirable, and considerable effort has been expended in attempts to control the power harmonics present in the plasma to produce a more uniform plasma, and thus a more uniformly processed wafer.
Processes to control the power harmonics present in a plasma are predicated on the wave nature of the AC electrical signal used to excite the plasma. In addition to the frequency of the AC signal described above, the signal has an amplitude, or strength, and a phase, or timing difference relative to other waves. The combination of several AC electrical signals of different frequencies, amplitudes or phases is governed by the principle of superposition. This principle states that the sum of two waves of differing amplitudes results in a wave that, in general, has an amplitude different from either of its addends. When waves are to be added, differences in their frequencies, phases or amplitudes can change the characteristics of the resultant wave. Thus, if AC electrical signals with different phases and frequencies are used in combination in a plasma processing system, the resulting excitatory waveform could demonstrate constructive or destructive interference.
Previous attempts to control the non-uniformity present in a plasma have included the use of a plurality of RF drive electrodes or alternately, a plurality of segments of a segmented electrode. The plurality of electrodes or segments are excited at a single RF frequency by means of a single RF oscillator and a plurality of separate RF amplifiers and phase shifters. An example of this technique is seen in U.S. Pat. No. 5,932,116 (Matsumoto).
Another approach to controlling the non-uniformity present in a plasma is found in U.S. Pat. No. 6,043,607. In this approach, a plurality of RF sources operating at a corresponding plurality of RF frequencies are used to generate a complex power waveform and excite a plasma within a semiconductor processing system. In the above-cited reference, the frequencies in the complex excitation waveform are not precisely controlled, so there are constantly varying phase differences between the plurality of RF sources.
Each of these previous attempts has focused on attenuating or accentuating the harmonic power frequencies present in a non-uniform plasma without independently controlling both the phase and amplitude of the input power at each selected harmonic frequency. In FIG. 1, the dotted-line surface profile 14 of the wafer 10 illustrates a potential advantage of a plasma processing system in which both phase and amplitude are controlled at each selected harmonic frequency. The non-uniform surface perturbations of the wafer 10 have been greatly attenuated in the dotted-line surface profile 14, resulting in a more uniform wafer.